This invention relates to a sensor chip, especially for measuring structures in a finger surface.
Fingerprint sensors based on capacitive coupling of an AC signal from the groove pattern of the finger to an array (matrix) of sensor elements are known from international patent application No PCT/NO98/00182. U.S. Pat. No. 5,963,679 describes a similar sensor with a different measuring principle and a two dimensional sensor matrix.
For special applications, e.g. when being mounted in a cellular phone or a lap-top PC, it is important that the sensor is made as small and light as possible. Such a miniature sensor may also be very cost effective if production techniques suitable for mass production are used.
Noise considerations are important when positioning the electronic interrogation circuit, often a costumer specified silicon circuit (“ASIC”) as close to the capacitance sensitive elements in the sensor array as possible, so that the length of the electrical conductors between the silicon circuit and the sensor elements is minimized.
The abovementioned requirements related to size and electronic functionality may be difficult to realize using standard packing techniques in which the silicon circuit is mounted in a plastic based or ceramic housing. At the same time as the housing effectively protects the sensor from the outer influences, such a solution may result in a sensor with relative large dimensions, while the housing “legs” gives an unsuitable interface to the finger.
In addition to these general problems it is strongly desirable to provide components adding extra functionality to the interface toward the user, to secure optimal signal quality. Thus it would be a great advantage if such components may be integrated into the sensor itself.
The object of this invention is to secure a cost effective and miniaturized sensor solution based on a naked silicon chip (ASIC) with a multi layer metallization having a surface defining the interface toward the user's fingers. The principle is based the fact that the array of imaging sensor elements is positioned in one of the upper metal layers on a silicon surface.
In a standard production process for ASICs, the silicon surface is provided with a number of metal layers with conductor leads, separated by thin dielectric layers. In addition there may be built a so called sandwich structure with extra layers of metal and dielectrics on top of the other layers. By laying the functional structures for detection of the finger's pattern in the upper layers of the ASIC, a miniature sensor as described above may be produced.
In U.S. Pat. Nos. 6,069,970; 5,862,248; and 5,963,679 fingerprint sensors are described being based on the abovementioned techniques. These solutions do, however, differ substantially in construction, so that the functions of each layer are different. These solutions relate to very complex two dimensional sensor structures in which the impedance is measured locally with drive electrodes for each pixel. This is, however, making strict requirements to the electronic circuitry in general.
The present invention relates to a simplified solution to the challenges described above.
Using a stimulation electrode in electrical contact with the finger an impedance measurement is obtained through the finger and the surface to a sensor electrode. This, in addition to the substantially linear sensor structure, thus provides a largely simplified sensor relative to the known solutions which is simple to produce and implement in small, mobile apparatuses.
Since the sensor thus may be produced using standard processes for IC production, this will give a very cost efficient sensor being suitable for mass production.